Circuits operating at high voltage are typically subject to voltage breakdown at higher elevations (lower atmospheric pressure conditions). Such voltage breakdown, manifested as an arc, is often harmful to the equipment involved. One solution to the problem is to use sufficiently high quality components so that either breakdown does not occur at altitudes of interest or that such a breakdown causes no harm. In some situations, equipment need not operate at higher altitudes, but need only remain undamaged so that it will be operational when the equipment is once again at lower altitudes.
An airborne weather radar is an example of such equipment. Specifications require that it must operate at relatively lower altitudes after being on, but not necessarily operational, at higher altitudes. It has become common to utilize ordinary television receiver circuits as the display portion of the radar. A television receiver contains a high voltage power supply which typically produces damaging voltages at higher altitudes (low pressure). U.S. Pat. No. 4,041,357, issued Aug. 9, 1977, to the instant inventor and assigned to the common assignee describes a power supply circuit in which a pressure sensitive switch operates to shunt voltage produced by the high voltage power supply thereby reducing its output voltage to safe levels. Such a system requires that certain parts of the high voltage power supply, for example, the power transformer, be accessible to effect the shunting arrangement. When such parts are not accessible, another protection circuit must be utilized.
Changes in environmental parameters additional to or instead of low pressure may also cause power supply damage if the power supply continues to operate at its normal output conditions. Thus, for example, power supply arcing may occur under conditions of high humidity. A power supply or other associated circuits may also be damaged by elevated temperatures.